1. Field of the Invention
This invention relates to an injection device and in particular, but not exclusively, to an injection device for use with dual or multi-chamber cartridges.
2. Description of the Related Art
In typical dual chamber cartridges two spaced bungs are disposed within the cartridge to define two separate chambers, for example a forward chamber containing lyophilised powder and a rearward chamber containing diluent liquid. A bypass channel in the cartridge wall provides a bypass path between the two chambers, but is initially closed by the presence of the forward bung. In use, as pressure is applied to the rearward bung to move it, the liquid in the diluent chamber and the forward bung move together forwardly and in doing so uncover the bypass channel so that the fluid from the diluent chamber flows into the forward chamber to mix with the lyophilised powder. The forward bung and the rearward bung may be of similar form or the forward item may be in the form of a moveable wall.
In such cartridges, it will be appreciated that two phases of movement of the rearward bung are required, namely a first, reconstitution phase during which the rearward bung moves to move the liquid in the rearward chamber forward to mix with the material in the forward chamber, and a second, dosing phase during which metered movement of the rearward bung is required to expel a preset dose.
In a typical pen injector such as our AUTOPEN® device, the pen injector comprises a main body housing the pen injector mechanism which screws into a cartridge holder which holds the cartridge of therapeutic substance. In use the user dials in a required number of units dosage on a dose selector on the body and then presses the release button which causes an internal plunger in the body in contact with the bung to advance it by the required amount to expel a corresponding dose from the cartridge in the cartridge holder.
A problem arises in relation to such devices in relation to use with dual chamber cartridges because the amount of movement required during the reconstitution phase 15-20 mm or more is much greater than that required for the doses (typically 0.1 to 0.2 mm per unit dose). If the pen injector mechanism were to be used to try and achieve the length of stroke required for the reconstitution phase this would be extremely tedious requiring many cycles of repetitive dialling in and firing. A previous attempt to overcome this problem provided an pen injector in which the body and the cartridge holder had extended threaded mating sections so that the holder could be engaged with the body and screwed continuously until the plunger had driven the rearward bung by an amount sufficient for the reconstitution phase. Thereafter the pen injector mechanism could be operated as required to effect a series of doses. This extended screw thread arrangement is also extremely tedious and time consuming.
There is therefore a need for an injection device for use with a multi-chamber cartridge, to allow convenient reconstitution followed by the usual dose metering in successive injections.